Interference due to channel dispersion presents one challenge to obtaining high data transmission rates in Code Division Multiple Access (CDMA) systems, such as Wideband CDMA and IS-2000. Performance in CDMA systems is sensitive to multi-path dispersion when a low spreading factor and/or multicode is used to transmit data. With dispersion, multiple echoes of the transmitted signal arrive at the receiver with different relative delays. These echoes interfere with one another. The interference results in a loss of orthogonality between successive symbols and between symbols sent on different, orthogonal codes.
Generalized RAKE (GRAKE) receivers provide one means for suppressing interference. Interference suppression is achieved by treating Intersymbol Interference (ISI) and Multiple Access Interference (MAI) as colored Gaussian noise. The noise correlation across fingers is exploited by adapting the finger delays and combining weights. In this way, the orthogonality between user signals may be partially restored. Recently, further improvements in GRAKE receivers have been proposed for the High Speed Downlink Packet Access (HSDPA) mode of WCDMA that take into account waveform correlations.
Multicode detection techniques that rely on waveform correlations provide another technique for suppressing MAI and ISI. Exemplary multicode detectors include a Maximum Likelihood Sequence Estimation (MLSE) detector, a minimum mean squared error (MMSE) detector, and a decorrelating detector. Because the computational complexity of conventional multicode detectors grows exponentially with the number of codes, there remains an interest in reducing the computational complexity associated with multicode detection.